1. Field of the Invention
The present invention relates to an optical head apparatus for detecting a radial tilt of an optical recording medium and an optical information apparatus for performing at least one of recording and reproducing operations using such an optical head apparatus.
2. Description of the Related Art
Generally, in an optical recording medium such as a recordable or a rewritable optical disk, a tracking groove is formed in advance only for a tracking operation. That is, a tracking error signal is calculated by the tracking groove using a push-pull method, so that the location of an objective lens is changed in accordance with the tracking error signal. In this case, if the objective lens is shifted in the radial direction of the optical recording medium, a so-called lens radial shift offset occurs in the tracking error signal.
A first prior art optical head apparatus is known to reduce the above-mentioned lens radial shift offset, thus suppressing the deterioration of the read/write characteristics due to the lens radial shift offset (see: JP-A-2000-082226).
On the other hand, in an optical recording and reproducing apparatus including an optical head apparatus, the recording density is inversely-proportional to the square value of the diameter of focused spots formed on an optical recording medium. That is, the smaller the diameter of focused spots, the larger the recording density. Also, the diameter of focused spots is inversely-proportional to the numerical aperture of an objective lens. That is, the larger the numerical aperture of the objective lens, the smaller the diameter of focused spots.
When the optical recording medium is inclined in the radial direction with respect to the objective lens, the configuration of focused spots fluctuates due to the coma aberration arisen from the inclination of the optical recording medium in the radial direction which is called a radial tilt, so that the read/write characteristics deteriorate. The coma aberration is proportional to the cubic value of the numerical aperture of the objective lens. That is, the larger the numerical aperture of the objective lens, the narrower the margin of the radial tilt of the optical recording medium for the read/write characteristics. Therefore, in an optical recording and reproducing apparatus having an optical head apparatus with a larger numerical aperture objective lens, it is required to detect and compensate for a radial tilt of the optical recording medium.
A second prior art optical head apparatus is known to detect and compensate for a radial tilt of an optical recording medium (see: Ryuichi Katayama et al., “Substrate Thickness Error and Radial Tilt Detection Using 5-Beam Optical Head”, Optical Data Storage Topical Meeting 2001, pp. 97-99, April 2001). This will be explained later in detail.
In the above-described first prior art optical head apparatus, it is impossible to detect and compensate for a radial tilt of the optical recording medium.
On the other hand, in the above-described second prior art optical head apparatus, it is impossible to reduce the lens radial shift offset in the tracking error signal, so that the read/write characteristics deteriorate. Also, since five focused beams each requiring four photodetecting portions, i.e., twenty photodetecting portions are used, a control unit including current-to-voltage conversion circuits connected to the photodetecting portions and calculating circuits becomes complex, which would increase the manufacturing cost.
A third prior art optical head apparatus is known to reduce the lens radial shift offset in the tracking error signal as well as to detect and compensate for a radial tilt of the optical recording medium(see: Ryuichi Katayama et al. “Radial Tilt Detection Using 3-Beam Optical Head”, Proceedings of SPIE, Vol. 4090, pp. 309-318, 2000). That is, three focused beams i.e., a main beam and two sub beams are used, and two photodetectors are required for each of the three focused beams.
In the above-described third prior art optical head apparatus, however, since a radio frequency (RF) signal is obtained from only one photodetector for the main beam, the signal-to-noise (S/N) ratio of the RF signal is low.